The chapel holds tremendous significance for the royal family: Louis' elder brother Prince George was also christened there, and the chapel was where the late Princess Diana's body was laid before her funeral.
Other guests included Louis' new aunt Meghan Markle, who wore a green Ralph Lauren outfit with a Stephen Jones hat alongside her husband of two months, Prince Harry. For the religious ceremony at St. James's Palace, also in London, all eyes were on that elaborate Prince Louis christening gown, plus his royal parents Kate Middleton and Prince William, as they accompany him to the event at the Chapel Royal.
It is a replica of the gown worn by Queen Victoria's daughter for her christening in 1841.
The three-year-old was walking hand-in-hand with her dad Prince William after the service at the Royal Chapel at St James's Palace told waiting photographers: "You're not coming".
Prince William and Kate Middleton opted for "O Jesus, I have Promised" and "Lord of All Hopefulness" as the hymns for the christening ceremony. The original is now too fragile to wear, so the queen commissioned the copy from her royal dressmaker, Angela Kelly.
What a magnificent (and elegant) way to welcome tiny Prince Louis to the faith and the royal family.
Welby baptised Louis by pouring holy water from the River Jordan over his head.
His younger sister, Princess Charlotte, looked a little more interested in the day's events - even if she did have her fingers in her mouth.
Mr. Nicholas van Cutsem is one of William and Harry's oldest friends.
After the service, the royal couple hosted their guests for a tea party in Clarence House, where, as is tradition, they served up cake from their 2011 wedding.
Mr. Harry Aubrey-Fletcher attended both Ludgrove prep school and Eton with William.